Background: Mycosis fungoides (MF) can begin as early as the first decade of life. Few studies have reviewed MF in younger patients and none has been large enough to assess prognosis and outcome. Objective: We reviewed the clinical characteristics, prognosis, factors related to disease progression, and therapy in patients with MF younger than 35 years of age. Methods: Fifty-eight patients were entered into this retrospective cohort analysis. Results: Significantly fewer patients with MF who are younger than 35 years presented with erythroderma (T4) and more with limited patch/plaque (T1) disease than older patients. Duration of skin disease before diagnosis of MF did not vary between the two groups. The long-term survival of younger patients with MF is significantly decreased when compared with a race-, age-, and sex-matched control population (p < 0.001). Disease-specific survivals (DSS) of younger and older patients are similar, but young patients show a slight but significantly better overall DSS (p < 0.02). However, DSS comparison of generalized patch/plaque (T2) and tumor stage (T3) patients with MF showed no significant difference between young and old patients (p = 0.47, p = 0.59). Patient age was not a significant predictor of survival when controlled for T-stage. Sixteen of 58 young patients with MF have died, 13 because of MF (22%), compared with 138 of 500 older patients (28%) who died as a result of MF. All younger patients with MF who progressed had at least T2 disease at presentation. Fifty of 56 young patients with MF and T1-T3 disease were treated initially with total skin electron beam or topical nitrogen mustard. The response to therapy was similar in younger and older patients with MF. Conclusion: T1 disease is more common and T4 disease is unusual in young patients with MF compared with an older population of patients with MF. Young patients with T1 disease, all of whom were treated with either topical nitrogen mustard or total skin electron beam therapy, or both therapies, showed no disease progression. Overall, young patients with MF showed slightly better DSS, but this was because of differences in stage distribution.(J Am Acad Dermatol 1998;38:696-701.)